1. Filed of the Invention
This invention is directed to methods for patterning carbon nanotube coatings and carbon nanotube wiring made by the methods.
2. Description of the Background
Current electronic devices, including semiconductor-based devices as well as wiring circuits of larger scale, rely on conventional wiring technologies that use metal wiring lines or high impurity regions formed in a semiconductor substrate. Semiconductor-based devices have metal wiring layers that are formed on the semiconductor substrate and interconnect device elements formed on the surface of the semiconductor substrate. The metal layers themselves are often interconnected by via holes piercing through insulating layers separating the metal layers. In addition, portions of the semiconductor substrate that are doped with impurities function as wiring lines within the elements formed on the surface of the semiconductor or between these elements.
Although these wiring lines are made extremely fine using modem photolithographic technologies and, thus, the semiconductor-based devices are made compact, the manufacturing processes of such wiring lines require film forming and manipulating techniques that are operable only in high vacuum. For example, metals such as aluminum and copper are formed on the semiconductor substrate using physical vapor deposition techniques including sputtering and evaporation. Impurity ions such as boron and phosphorus are injected into the semiconductor substrate using ion implantation techniques to form conducting portions in the substrate. Amorphous silicon layers are formed on the substrate by chemical vapor deposition techniques and later transformed into polysilicon by annealing to form a wiring layer. Many of the layers and films formed as above must be patterned to a predetermined wiring pattern by etching process such as reactive ion etching. The level of vacuum may vary depending on the methods, for example 10−6 torr (sputtering) to a few torr (reactive ion etching). Whatever the vacuum level is, the installation and maintenance of such instruments are expensive. Furthermore, all of the wiring lines formed by above methods do not transmit light well with an exception of those made of inorganic electrode materials such as indium tin oxide (ITO). Extremely thin metal films may be translucent, but stacking of such films results in forming of a layer that practically blocks light. A transparent ITO film may be formed relying on the high level vacuum instruments, but is not flexible due to its inorganic nature. Furthermore, the supply of indium is limited.
Wiring circuits of larger scale are fabricated using methods that do not require expensive installation or maintenance of manufacturing instruments. Print circuit boards are fabricated by etching copper clad laminates coupled with print techniques. These print boards may be rigid when the board is based on epoxy/glass laminates, and may be flexible when it is based on polyimide laminates. Similar structures are made by printing conductive pasts directly on a substrate. The conductive ingredients of the pasts are typically metal fillers such as silver. The conductive pasts are printed on the substrate using screen printing technique or the like. When performance requirements of wiring circuits are very low, the pastes may be applied by a brush.
Though these fabrication methods are inexpensive, it is not possible to make compact device, such as a semiconductor device, relying on these methods. Furthermore, the wiring lines made by these methods are not transparent. Light is blocked by copper clad in the laminate structure and the silver paste applied on a substrate. Accordingly, the wiring structures made by these methods are not applicable to devices that require fine patterning of transparent conductive film, such as electroluminescent display device and liquid crystal display device.
Efforts have been made to provide transparent electrodes to replace ITO film. A typical example is a suspension of ITO particles in a polymer binder. However, this ITO filled system cannot match the electrical conductivity of a continuous ITO film. Furthermore, transparent conductive polymer materials are now being developed. These polymers typically require dopants to impart conductive properties, and are applied on a substrate using screen printing or ink jet application technique. Although they are still at a development stage and yet to reach the conduction level of a ITO film, the presence of dopants is expected to have an adverse effect on controlling the conductive properties, and may not be compatible with device miniaturization.
Films made of carbon nanotubes are known to have surface resistances as low as 102 ohms/square. U.S. Pat. No. 5,853,877, entitled “Method for Disentangling Hollow Carbon Microfibers, Electrically Conductive Transparent Carbon Microfibers Aggregation Film and Coating for Forming Such Film,” describes formation of such conductive carbon nanotube films, and U.S. Pat. No. 6,221,330, entitled “Processing for Producing Single Wall Nanotubes Using Unsupported Metal Catalysts,” generally describes production of such carbon nanotubes used for forming the conductive films. However, there have been no report in the art on a method for patterning the film made of carbon nanotubes.
Coatings comprising carbon nanotubes such as carbon nanotube-containing films have been previously described (see U.S. patent application Ser. No. 10/105,623, which is incorporated herein by reference). For example, such films may have a surface resistance as low as 102 ohms/square and a total light transmittance as high as 95%. The content of the carbon nanotubes in the film may be as high as 50%.
It has been surprisingly discovered that such materials can be formed by a two step method, which results in carbon nanotube film that have a low electrical resistance as well as a high light transmittance. First, a dilute water solution of carbon nanotubes is sprayed on a substrate, and water is evaporated leaving only the consolidated carbon nanotubes on the surface. Then, a resin is applied on the consolidated carbon nanotubes and penetrates into the network of the consolidated carbon nanotubes.